1. Field of the Invention
The invention relates to a method for operating a gas engine, in particular a stationary gas engine, to an apparatus having a gas engine, in particular a stationary gas engine, and to a vehicle, in particular a commercial vehicle, for carrying out the method and/or having the apparatus.
2. Description of the Related Art
The operation of a gas engine running on a carbon-containing fuel gas using a very lean fuel gas/air gas mixture (e.g., air/fuel ratio λ=1.7) is known. In this way, nitrogen oxides (NOx) emitted by the gas engine are kept low. However, thermodynamic conditions dictate that a very lean mode of operation of the gas engine can result in a significant reduction in the efficiency of the gas engine. Moreover, it may be necessary in the case of a gas engine operated on a lean mixture to provide a nitrogen-oxide-reducing exhaust gas aftertreatment in an exhaust line adjoining the gas engine, e.g., if the nitrogen oxides (NOx) emitted by the gas engine exceed the legal limits. The usual practice with an engine operated on a lean mixture is to use an active NH3-SCR catalyst (Selective Catalytic Reduction) as an exhaust gas aftertreatment element for NOx reduction, by which the level of nitrogen oxides (NOx) in the exhaust gas flowing through the SCR catalyst is lowered using ammonia (NH3) as a reducing agent. Here, the ammonia is usually introduced into the exhaust line as an aqueous urea solution between the gas engine and the SCR catalyst, as viewed in the direction of flow of the exhaust gas, since the exhaust gas flowing directly out of the gas engine does not normally contain any ammonia. However, feeding in ammonia in this way is an involved process because of the provision of additional injectors, additional tanks and metering control (including sensors) and is expensive particularly because of the continuous consumption of ammonia.
Moreover, the practice of operating a gas engine running on a carbon-containing fuel gas in accordance with a Miller cycle is also known. In this Miller cycle, the intake valves of the gas engine are closed very early. As a result, the charge in a cylinder of the gas engine is initially expanded, resulting in a decrease in the temperature in a combustion chamber of the cylinder. A decrease in the temperature in the combustion chamber leads to a rise in the knock resistance of the gas engine. As a result, the compression ratio of the gas engine and thus the efficiency of the gas engine can be increased.
When using the Miller cycle, it is usually necessary to impose a high boost pressure on the combustion air fed to the gas engine or on the fuel gas/combustion air gas mixture fed to the gas engine. This high boost pressure or positive scavenging gradient across the combustion chamber leads to an increase in the slip of unburnt hydrocarbons (CyHz) in gas engines with external mixture formation, as a result of which a large quantity of hydrocarbons (CyHz) is emitted by the gas engine. This slip of unburnt hydrocarbons (CyHz) can be reduced by reducing the valve overlap (time during which both the intake valves and the exhaust valves of the gas engine are open). However, reducing the valve overlap generally causes a drop in the efficiency of the gas engine.
In summary, it can therefore be stated that a gas engine operated with a lean mixture and according to the Miller cycle cannot be operated at the maximum possible efficiency owing to the reduced valve overlap. Moreover, the use of an NH3-SCR catalyst to lower the level of nitrogen oxides (NOx) emitted by the combustion engine has the disadvantages already mentioned.